The invention relates to a method and an apparatus for detecting the motion of an element relative to a sensor apparatus, in particular for detecting the angle of rotation of a rotating element by way of a sensor.
Various embodiments of such sensor apparatuses are already in use, for instance in vehicles. For instance with so-called Hall elements as rpm sensors on the wheels for an anti-lock brake system (ABS), as rpm and phase transducers for engine control, or as steering angle sensors for so-called vehicle dynamics control, and for electrical steering aids.
Among the most essential demands made of these rpm sensors, both in ABS and in the field of engines and transmissions, are as large an air gap as feasible and high immunity to vibration. However, it must be noted that a highly sensitive sensor is intrinsically also highly sensitive to excitation by the vibration that impairs the outcome of measurement and that briefly also causes changes in the directions of rotation.
From German Patent Disclosure DE 197 50 304 A1, for instance, an apparatus for contactless detection of an angle of rotation by means of a pulse wheel is known, in which direction of rotation detection is possible by providing that in one direction of rotation, a change in the spacing of the pulse-tripping elements has been made, thus changing the duty factor.
To minimize the sensitivity of such sensor apparatuses to vibration, in conventional rpm sensors, a variable hysteresis is often also employed. This requires that first the signal amplitudes be measured, and the hysteresis is then adapted flexibly to them. For large input signals, major hysteresis is then selected, while a correspondingly reduced hysteresis is selected for small input signals; that is, when the air gap is small, the amplitude required for switching is increased. A substantial disadvantage of this method is the loss of immunity to air gap impacts during operation, which can briefly cause a major reduction in the signal amplitude. A previously increased hysteresis at the switching point of the sensor, signal loss can then occur in the event of an air gap impact.
Moreover, this method can be employed only once calibration of the sensor has been done, since it is only after calibration that the signal amplitude is known. Immediately after the sensor is switched on, the vibration sensitivity is still unchanged.
From U.S. Pat. No. 5,451,891, it is for instance known to use an adaptive hysteresis that is dependent on the signal amplitude. In this case, a coupling factor is determined, as a quotient of the measured sensor amplitude and the frequency, and based on this coupling factor, the hysteresis is adjusted in proportion to the production of the coupling factor and the frequency. With this known method, only the behavior of passive sensors can be compensated for; such sensors furnish a very small signal for low excitation frequencies, and output a very high amplitude for high frequencies. However, the behavior of sensors that regardless of the signal frequency furnish a constant internal signal amplitude cannot be improved.